The user interface of a telephone, particularly in a car radio telephone, normally comprises a receiver part and an operating section (the keypad section). The receiver and the operating section may be combined or may be separate. When they are combined the operating section is often located on the rear of the receiver. The assembly comprising the receiver and the operating section is here called a handset (HS, Hand Set).
Normally the operating section is connected via a cable to a bracket having a cable connection to the subscriber unit, in a car radio telephone to the radio unit or the transceiver unit of the car radio telephone. In a car radio telephone the receiver and the operating section of the telephone are normally connected via a cable to the radio unit also when they are separately located. In a conventional telephone the operating section of the telephone is normally located in the subscriber unit. Mobile phones are becoming increasingly common, in business as well as in private use, and they are used in cars and boats. A drawback of mobile phones has been the connection of the transceiver unit to the handset via a cable, which ties the user to the site of the radio unit when the user desires to make and/or to receive calls. For instance in a car the handset is further fixed by the cord to the neighbourhood of the front seat in the car, whereby persons sitting in the car's back seat experience difficulties using the phone.
The use of the phone was made more flexible when the receiver or the whole handset was replaced by a receiver or handset based on radio frequency or infrared/ultrasonic communication. It is known to use wireless handsets in subscriber sets operating in the conventional public telephone network, whereby the communication between the handset and the subscriber set usually operates on an analog RF link. Wireless handsets usually operate on RF channels at frequencies below 50 MHz and with a power below 100 mW. Different handsets all operate approximately at the same frequencies, and therefore interference between handsets will occur. These wireless handsets further receive interference from television, police and other radios operating at the same frequencies. U.S. Pat. No. 4,745,632 presents a wireless mobile phone system, which combines a conventional mobile phone and a conventional subscriber set comprising a wireless handset. Certainly this is a solution requiring much space, when there are two pairs of interconnected handsets and subscriber sets. U.S. Pat. No. 4,659,878 presents a mobile phone comprising a wireless handset, in which the connection between the handset and the radio unit is an RF connection without interference. One disadvantage of an RF connection is certainly that at present a car contains a large number of different electronic devices, and thus much electromagnetic radiation or radio pollution is created. Further, if the car contains a large number of devices using RF connections they easily cause mutual interference.
European Patent Application, EP-A-3 165 058 A presents a telephone having a receiver in wireless analog infrared communication with the subscriber set. A wireless connection as a secondary embodiment between the handset and the transceiver is shown in U.S. Pat. No. 4,219,411. Here the primary connection is a cable connection, and if the connection is wireless, then the operation has to be controlled by a direction switch. The connection is primarily operated by the cable, even if the patent mentions the wireless as one possibility. The wireless connection in the first place is conceived for the control operations of the telephone, and not for speech transmission. This is mainly due to the fact that difficulties occur when attempting speech transmission over an infrared connection.
There has been a problem in providing sufficient infrared power in the car cabin so that the wireless device would operate regardless of its location in the car. A limiting factor in achieving a sufficient infrared power is the maximum continuously emitted power which the infrared LEDs can provide. In analog infrared connections the continuous power demand is often so high that the operating time will be short for a handset operating with a battery. EPO Finnish patent No. FI-82334 (corresponding European patent application EP 0 383 277) presents a system having a separate wireless receiver and a separate wireless operating section. In this system the operating section is unidirectional, i.e., because it has no receiver it can only issue control commands.